Substituted thickener lubricating grease



- A. F. SlRlANNl ETAL 2,583,603

2 SHEETSSFEET 2 Jan. 29, 1952 SUBSTITUTED THICKENER LUBRICATING GREASEFiled Dec. 18, 1948 x 5 0 w M 5 5 N P 2 m 2 W a 1 5 My 2 y Um i mu 1 W0m m w 5 5 ww giik figure 5 Patented Jan. 29, 1952 UNITED STATES PATENTOFFICE SUBSTITUTED THICKENER LUBRICATIN G GREASE Application December18, 1948, Serial No. 66,130

9 Claims.

This invention relates to substituted thickener lubricating grease, andmore particularly, to an improved lubricating grease wherein thethickening or grease structure forming material is of a new type, suchas an aqueous gel, and preferably inorganic aqueous gel, origin.

In the prior art, lubricating greases have commonly been prepared byincorporating into mineral base lubricating oils of appropriate grade agelling or thickening agent capable of setting up with lubricating oilinto a grease-like structure. As a rule, the metal soaps of the fattyacids have been used almost universally as thickening agents. It hasbeen suggested that various other materials than soaps, such as certaincarbon blacks, bentonite, clay, aliphatic or aromatic sulfonates, andthe like, might be substituted in whole or in part for the soap as athickening agent. It has also been suggested that oils might bethickened to grease-like body by incorporating therein certain types ofinorganic gels. The prior art has further suggested that the water ofgelation in certain inorganic gels such as silica gel is preferablyreplaced without drying out the gel, since shrinkage results when such agel is dried, making it extremely difficult to convert the gel intogrease by substituting lubricant for the water. In other words, it isconsidered desirable, according to the prior art, to replace theoriginal liquid continuous phase by another liquid which is moresuitable for the intended use. This, it is suggested, may beaccomplished by using a series of mutual solvents. For example, theprior art suggests that an aqueous silica gel may be used as aningredientof a grease by first replacing the water by acetone, theacetone subsequently to be replaced viith textile spirits, and thetextile spirits finally replaced by a petroleum base lubricating oil.Such replacements may be accomplished by first extracting the liquidphase of the gel, i. e., water with one or more mutual solvents inseries and then extracting the latter mutual solvent by contacting withan excess of the lubricant. The object of the present invention is toimprove and extend the prior art methods and products by makingpossible, in some cases, a direct replacement of water with oil and alsoby stablizing or otherwise improving various properties of the resultingproduct, such as its resistance to moisture, and the like.

During the course of experiments relating to this invention, it has beenfound that conventional colloidal suspensions of grease-formingparticles, such as the soap particles used in the prior art, having ahigh ratio of length to breadth when recrystallized from oil. This meansthat they are long and slender, and relatively fibrous in in character.This appears to be particularly true with the sodium and the calciumsoaps that are widely used in greases It has also been found that strongthixotropic properties are imparted to calcium soap-oil compositions bythe addition of water in small quantities to such greases. As is wellknown in the prior art, it has been long cons'idered necessary to addsome water to the calcium soap greases to stabilize them.

While the exact bearing of the shape of the solid particles on theeventual viscosity of the colloidal composition is not known, it isbelieved that such shape is a factor of considerable importance in theformation of greases and greaselike compositions For example, withrespect to dispersions of particles approaching molecular sizes,Einstein's formula n=2.5 C where n is specific viscosity and C is volumeconcentration, has been found to hold true for dilute solutions ofmolecules which are considered generally spherical in shape. Thisrelationship changes to where j is the ratio of length to width.Furthermore, where the particle is thought to be of disclike shape, theformula applicable is where i represents the ratio of diameter tothickness of the disc. The particles of colloidal size are of coursemuch larger, but these formulae apply in a general way.

The above formulae suggest that particle shape is an importantconsideration when the purpose of the solid phase in a suspension is tochange the flow characteristics of the liquid phase. It appears alsothat in some cases a small amount of water is a useful grease structuremodifier. In conventional greases, it has been noted that there isconsiderable difference in the penetration value of greases havingsimilar soap content, depending upon the method by which the soap isintroduced into the oil. This suggests that the fineness of subdivisionas well as ultimate particle size of the solid phase is of considerableimportance. Apparently the maximum thickening effect commonly occurswhen the elemental particles of the solid are in the colloidal sizerange. Hence, it might be inferred from the preceding that the mostdesirable particle to be used as the solid phase in preparinglubricating grease may be compared to a flexible rod-like structure ofessentially one dimensional character or an infinitely thin disc. Themaximum thickening efiect appears to be produced by thin elongated orflattened elements.

For complete control of grease structure the fiber length of the solidphase should be controlled as far as possible. The solid phase shouldalso have a high melting point and as far as practicable it should behydrophobic to give water resistance to the composition. At the sametime it should attract and hold oil to its surface so as to preventseparation of the oil phase. Commonly available inorganic materials suchas silica gel, for example, are deficient in the latter property and itis an object of the present invention to overcome such deficiency.

The present invention involves, as one aspect, the principle that solidshaving the general characteristics mentioned above may be prepared bytwo general methods to control particle size. The particles may befinely divided and subdivided, as by grinding or other means ofcomminution, or they may be prepared by building up to the desired sizefrom the molecular sizes. It is frequently difiicult, however, to obtainthe particular range of sizes desired. For example, wood pulp whensuspended in Water may be ball-milled to a particle size approachingthat of soap crystals in conventional soda base grease, but ballmillingof this material in oil is practically impossible. The transfer of afinely comminuted material from one medium to another also is attendedwith difficulties. As suggested above, in connection with silica gel,the removal of Water from a suspension at pressures and temperaturesbelow critical permits the solid phase to shrink badly due to thecollapse of capillary spaces by the surface tension of the evaporatingwater. As a result, the solid becomes irreversibly coagulated and thewhole desired effect of the comminution is lost. Hence, the procedurementioned above of replacing the liquid phase without evaporation ordrying appears to be essential in the use of many materials of theaqueous gel type or analogous materials as grease thickeners.

The thixotropic structure which appears to be necessary for satisfactorylubricating greases appears to result from the greater adhesional forcesbetween discrete particles of the solid phase than between elements ofthe liquid and solid phase. These forces can be completely destroyed inmany cases by contaminating the surfaces of the solid particles. Inother cases these forces can be substantially increased by proper choiceof contaminants. For example, water may be used for increasing suchadhesion, whereas fatty acids added to titanium oxide produce theopposite effect. An'aspect of the present invention is a means ofcontrolling the forces mentioned, although the exact degree of controlis not specifically determinable.

This property of thixotropy is dependent apparently on the mutualdispersion of two separate phases into each other and is not simply afunction of particle shape of the normally solid phase. Particle shapebecomes of great importance, however, when it is desired to keep thetotal quantity or volume of solid low with substantial thickeningeffect. In other words, a high degree of mutual dispersion of liquidphase and the solid material is essential.

Hence particles having the greatest degree of aniso-dimensionality aredesirable. Such substances as ball-milled sand, finely divided commonsalt, and the like possess good thixotropic properties but theirparticle shapes are such that very substantial quantities are requiredfor a desired thickening effect. On the other hand, relatively longfibered materials such as ballmilled asbestos, wood pulp, and the likeare quite satisfactory in thickening power, smaller relative quantitiesbeing required. Hence the criteria for suitable thickening agents dependin part upon their particle shape and in part upon the forces ofattraction involved, although of course their relative afiinities foroil, their chemical and abrasive properties, and the like, are also ofgreat importance.

The inorganic gels such as silica gel, and analogous voluminousprecipitates of inorganic nature appear to be particularly desirable.The finely divided precipitated materials such as silica gel, Fe(OH)3,Al(OH)3, etc., appear to have the desired particle shape, the requisiteaffinity for oil, and low abrasive properties. Organic materials such ascellulosic fibers and viscose, and metallo-organic materials such as thesoaps are satisfactory. Many inorganic materials such as talc, bentonitetitanium oxide, carbon black, even grapdtef'and the like, have someutility, provided they can be processed so as to hold the oil in thegrease structure and resist its displacement by non-lubricating fluids,especially by water. The latter is an important aspect of the presentinvention and is accomplished in many cases by providing a suitablecoating for the solid phase at the dineric interface.

It is another object of the present invention to make possible theutilization of various inorganic or naturally occurring materials as thethickening or grease-forming ingredients of lubricating greases, so asto produce solid or semisolid lubricants or greases having desirableproperties for specific uses. Some of these products appear to be quiteuseful under certain unusual conditions, i. e., of temperature, etc.,where conventional soap-thickened greases would not be usable.

A still further object of this invention is to improve methods forreplacing the normal liquid phase such as water of gelation, orequivalent fluid, in inorganic gels and the like, with a lubricatingmaterial such as mineral lubricating oil to prepare a grease orgrease-like material. In some cases, such replacement may beaccomplished by a single step and in others by a plurality of successivesteps, but the present invention contemplates the replacement by eitherone or more steps.

In one aspect of the present process, the liquid phase of an appropriategel, preferably an inorganic gel such as silica aqua gels or aprecipitate such as aluminum hydroxide is extracted continuously withacetone or some other volatile liquid which is mutually soluble in theliquid medium of the gel and in lubricating oil. This part of theprocess, which is'analogous to that .critical temperature and pressure.

described by Kistler in U. S. Patent No. 2,260,625, is continued untilthe original liquid phase, normally water, has been completely or verynearly completely displaced by the solvent. The resulting gel,containing a new liquid phase, is then dispersed in a proper amount oflubricating oil and the mutual solvent, which is preferably volatile, isthen flashed ofi". In the present case, however, the processingpreferably includes the provision of a water-repellent coating for thesolid phase at the dineric interface. The resulting grease may be passedthrough a colloid mill to give it the final desired stable structure. Bythis method, greases may be successfully prepared from an inorganic gellike silica gel, or from other voluminous precipitates such as aluminumhydroxide, ferric hydroxide, or from certain other naturally occurringmaterials, such as wet finely divided mica, bentonite, cellulose, andasbestos. In general, the concentration of the solid phase in the finalproduct will range between and by weight, based on the totalcomposition.

An alternative method of substituting the continuous liquid phase ininorganic gels or voluminous precipitates consists in freezing the geland subliming out the solvent at a very low pressure. This procedureobviates the shrinkage due to surface tension at the liquid interface,but for reasonable efficiency the solvent should have a high sublimationpressure and a low heat of sublimation. A similar result may be obtainedby removing the liquid phase at its These considerations limit thesolvents which may be used. The following examples will more fullyillustrate the invention and make clear its application to lubricatinggreases.

In the attached drawings there are shown graphically in Figures 1 to 5the pressure vs. flow characteristics of some of the grease orgrease-like compositions of the following examples.

Example 1 Using a silica aqua gel, a grease was produced by exchangingthe water in the aqua gel with ordinary ethyl alcohol and then heatingin an autoclave at a pressure higher than the critical pressure to atemperature above the critical temperature of the alcohol (about 245 C.at 63 atmospheres), pumping off the alcohol vapor and dispersing theaerogel so produced in mineral base lubricating oil. Stable greases canbe prepared by this method, using as little as 1.7 to 3.7% by volume ofthe aerogel. The particular gels have a structure of silica fibers whichapparently is something like that of felt and they have a very highspecific surface of about 600 square meters per gram. Oil is addeddirectly to the gel and the mixture is milled to produce the greasestructure.

Example 2 Results similar to those of Example 1 may be obtained byheating silica aqua gel in an autoclave above the critical temperaturefor water. The water vapor is pumped off and extracted with acetone andthe acetone replaced by mineral oil, flashing off the solvent to form S.S. U. at F. viscosity and a viscosity index of 40 flowing through a .78millimeter diameter orifice, under the pressures indicated. Thesecondcurve, indicated at I, shows the flow characteristics of a grease basedon the same oil but containing 5% by weight of the aerogel. The curvesindicated at 2 and 3 show the results of greases using 7 and 10%respectively of the same aerogel in same oil. Viscosity measurementswere carried out at 25 C. It will be 'noted that the yield value or thethickening effect of the gel decreased with decreasing concentration ofaerogel without great change in mobility.

In the case of conventional greases, sodium stearate base greases, forexample, it is commonly desirable to use oils of low viscosity index.With silica gel thickened greases, such as those of Examples 1 and 2,the greases have no true melting point and are not affected by theviscosity index of the oil. Hence oils of high viscosity index may beused. This ordinarily is not true of soap thickened greases. Inspectionof silica gel base grease with respect to abrasion indicates that thematerial is quite free of abrasive properties.

As previously suggested, natural silica gel greases are not entirelysatisfactory for some purposes, especially where appreciable quantitiesof moisture are present, because the hydrophilic surface of the silicapreferentially absorbs water which under moist or humid conditions willgradually displace the oil. Thereafter, on evaporation of the water thesilica will shrink to a point where it becomes useless as an oilthickener. Hence it is usually quite necessary to increase the waterrepellency of greases of this character.

Such water-proofing may be accomplished to a reasonable degree byproviding a water-proofing coating of the solid phase at the dinericinterface. This may be done by a thermal, vapor phase polymerization ofbutadiene and also of styrene on the surface of the silica aerogelpreferably before the oil is incorporated therein. For example, gaseousstyrene or butadiene may be passed over the dry aerogel in a heated tubeand this has been accomplished experimentally. The silica gel productsobtained in both cases were rather greasy to the touch and when mixedwith oil imparted good water repellent qualities to the grease. Testsindicated that this repellency continued when 20 grams of grease wereadded to 200 cc. of water at 70 C. and stirred mechanically for 15minutes. Qualitatively, some thixotropy appeared to be lost in this mix-It is not always necessary to pretreat the inorganic gel. Monomericstyrene was polymerized directly on to the surface of the silica gel byadding 2% of styrene to the oil before incorporating the silica gel andthereafter placing the grease prepared therefrom in an oven at 100 C.for two days. The resulting product was considerably darker than theoriginal oil but it had excellent water resistance without too much lossin thixotropy. Better results were obtained by adding to 2% ofpolystyrene dissolved in chloroform, directly to the grease, followed'byflashing off the chloroform. The latter experiments seem to indicatethat both monomeric and polymeric styrene are adsorbed by silica,although this is not absolutely certain.

A silicone type of water-proofing material may also be used, givingexcellent water-proofing but lowering the thixotropic propertiesconsiderably. In general, a compressed monolayer of a water repellent isdesirable on the silica surface to give protection without substantiallyinterfering with oil adsorption.

Aluminum stearate also may be used as the water proofing agent. Thismaterial may be added in small quantities, 0.25 to 1%, for example,based on the dry weight of the silica gel to give good water resistance,although it commonly results in some loss in thixotropy.

Figure 2 of the drawing shows the flow-pressure relationship of variousgreases, using an oil of 300 S. S. U. viscosity at 100 F. and aviscosity index of 40. In the case of Figure 2, commercial aerogel wasused as thickening agent in quantities of 10%, based on the weight ofthe total composition. Curve IA shows the flow properties of greasewater-proofed with silicone type waterproofing agent; curves 2A and 3Ashow respectively results of using 0.5% and 0.125% of aluminum stearatein the same grease; curve 4A shows the flow properties of the untreatedgrease. For some purposes, greases of the general character describedabove may be prepared by using a small quantity of a hygroscopic agentof low volatility, such as glycerol, to prevent coagulation as the Wateris withdrawn from the gel. The quantity of glycerol required is of theorder of about 1% to 2% by Weight, based on the final grease product.

Example 3 Another inorganic material of thickening power approximatelyequal to that of silica is aluminum hydroxide. This material is somewhatdeficient in water resistance, but not as objectionable in this respectas silica. It has been found more difiicult to waterproof greasesthickened with aluminum hydroxide than silica. Small quantities ofstearic acid, for example 0.25% to 2.0%, based on the weight of thetotal composition, appear to improve water resistance of aluminumhydroxide greases, probably due to the formation of minute quantities ofalumi num stearate at active centers, probably over the dineric surfaceof the solid phase. The other water proofing agents, such as polymerizedstyrene mentioned above, appear also to be applicable to aluminumhydroxide base grease, also reaction products of polybasic acids andpolyhydric alcohols.

Example 4 material is used, it appears to be necessary to completelyremove the water before flashing ofi the solvent since otherwise thewhole system undergoes a change in phase and the mass becomes liquid.

Example 5 Natural fibers of cellulose, collagen and asbestos, whenreduced suficiently in size by wet ball milling followed by solventexchange to remove the water and replace it with oil, produce greases ofgood appearance at low concentrations of thickener. Ball-milling overextended periods tends to shorten such fibers considerably which isobjectionable. It is preferable to use other methods whichwill notunduly break up the fibers since the ratio of fiber length to diam--eter is of some importance as indicated above. Water resistantproperties may be imparted in the same general manner as described abovein connection with Examples 1 to 3.

Example 6 Viscose precipitated with sulfuric acid, washed and solventexchanged as described above, produces a reasonably good grease with lowcellulose concentration. The particles, however, tend to have too smalla length to width ratio. Hence the grease is not of particularly goodstability. A material of elongated fibrous structure in the solid phaseis preferable.

Example 7 An alcogel of calcium acetate produced a good grease after oilwas substituted by solvent exchange. The weight of dry gel constitutedabout 6% of the final product. The needle-shaped crystals had a tendencyto grow, however, with the result that the grease lost consistency onstirring. The experiment shows, however, that the principle of solventexchange is applicable to soap thickened greases.

Example 8 An aqua gel of sodium stearate was solvent exchanged withoutheating and incorporated into lubricating oil in a 5% concentration byweight. The mutual solvent employed was acetone. The resulting productwas approximately equivalent in consistency to a high melting soda soapgrease having a soap concentration of 15% by weight.

Various precipitates of voluminous type appear to have certain utilityin the general process described above. Thus, certain volcanic or othercollodial clays, such as bentonite and related materials, have theproperty of swelling in water to produce a heavy paste of low solidsconcentration. Some of these materials are not satisfactory for makinglubricating greases because they show shrinkage when attempts are madeto exchange the solvent. Apparently a threedimensional structure doesnot exist in greases produced from these disc-like or plate-likematerials to the same extent as with rod-like structures and to thatextent they are less satisfactory.

Certain materials, such as suzorite and vermiculite, two minerals of themica type, gave fairly good greases after extensive wet ball-milling andsolvent exchange. A variety of other materials which appear to havepossibilities are lignin, graphite, talc, magnesium oxide, calciumcarbonate, ethyl cellulose, sodium carboxy-methyl cellulose, Irish moss,silt and sodium algenate. The greases listed in the table are exemplary.

Table Per Cent Per Cent Sohd phase Conc. Conc. Treatment (volume)(weight) Commercial volclay... 7. 8 17 Swollen 311 water, then solventexange Suzorite 11. 2 29 Ball-milled, then solvent exchanged.

8. 2 23 D0. 9. 5 23 Do. 5. 6 15 Ball-milled 16 days, then solventexchange 6. 5 22 ppt. Then solvent exchanged. 4. 7 17 D0. 3. 3 8. 4 D0.3. 6 9. 1 Do. 1. 7 4. 9 aqua gel made, then solvent exchanged. 1. 8 5.2. 7 7. oil added directly. 3. 7 10 5. 0 5 aqua gel made, then solventexchanged. 4. l 6. 1 alcogel made, then solvent exchanged. 6 gball-1r)nilled, then solvent exchanged.

o. 29. 7 50 ppt. in acetone, then dried at 105.

These greases were all made using an oil of 300 S. U. S. viscosity at100 F. and 40 V. I.

Figure 3 of the attached drawings shows flowpressure relationships forsuspensions of carbon black and cuprene in a 300 S. U. S. viscosity oil(210 F.) having a viscosity index of 40. A large change in mobility withrelatively small changes in thickening efiect is shown by theconvergency of the several graphs IB to 4B in this figure.

Figure 4 shows flow-pressure relationships of various greases, that ofcurve 5 being prepared with asbestos ball-milled for 16 days. The greaseof curve 6 contained 17% of ferric hydroxide, Fe(OH)z. That of curve 1contained 9% of the aerogel referred to in Examples 1 to 3 and that ofcurve 8 contained 9% of aluminum hydroxide (A1(OH)3 precipitate.) Asuperior yield value or thickening eifect of the aluminum hydroxideshould be noted.

Figure 5 shows the flow-pressure relationship for silica aerogel basegreases, 10% concentration, by dry weight of gel, based on the totalcomposition in oil of 300 S. U. S. viscosity at 210 F. and viscosityindex of 95. The upper curve IC of this figure represents the greasetreated with 1% of polystyrene whereas the lower curve 20 represents theuntreated grease.

It will be apparent from the foregoing that the present inventioncontemplates broadly the use of various substitutes for soap asoil-thickening agents for greases wherein the lubricating oil isincorporated by substitution or by solvent displacement of the originalliquid phase. The invention contemplates particularly the waterproofingof such materials at the dineric interface so that oil incorporated intothe grease will not readily be separated. In general. it appears to bemost desirable to use a material of relatively long fibrous or thinplate-like structure and in concentrations of 1 to 20% by dry weight.based on the finished lubricant, and in most cases this material shouldbe waterproofed.

As a rule, the grease will contain 0.05 to about 7%, preferably 0.1 to1% by weight, based on the total composition, of a waterproofingmaterial such as silicone polymers, alkyd resins. aluminum stearate,hydrocarbon polymers such as polyst rene and the like. Specificcompositions co taining silica-type polymers and alkyd resins aredisclosed and claimed in our conending applications, Serial Nos. 66,131and 66,132, filed December 18. 1948.

Bv rease is meant a solid or semi-solid composition containing availablelubricating oil for efiicient lubrication, thickened sufiiciently toprevent substantial flow or los from bearings, gears and the like. Suchlubricant greases or grease- 5 like compositions while intended forlubrication primarily, may be useful for other purposes. They may, ifdesired, contain various modifiers, such as oxidation inhibitors,tackiness agents, extreme pressure additives, surface active agents andthe like, as will be understood by those skilled in the art. The mineralbas lubricating oils, as recited in some of the claims, may include suchmodifiers. The invention, however, is not necessarily limited to mineralbase oils, since other oils of the natural or synthetic ester type, suchas the alkyl esters of dibasic aliphatic acids, e. g., di-2- ethyl hexylsebacate, and the like, as well as the oily polyglycols, and the like,may be used in lieu of mineral base oil for many purposes. For some 4.0purposes, such may be superior, as for extreme low temperaturelubrication, for example.

What is claimed is:

1. A lubricating composition consisting essentially of mineral baselubricating oil thickened to a grease-like consistency by 1 to 20% byeral base lubricating oil thickened to a greaselike consistency by 1 to20% by dry weight, based on the total composition, of an inorganic gelof the group which consists of silica gel, aluminum hydroxide and ferrichydroxide having its normal water content substantially entirelyreplaced by said lubricating oil, and 0.05 to 2% of a polystyrenewaterproofiing agent covering said gel in a compressed substantiallymonolayer coating to r inhibit reabsorption of moisture by said gel. 3.Composition according to claim 2 wherein said gel is silica gel.

4. Composition according to claim 2 wherein said gel is silica aerogel.

5. Composition according to claim 1 wherein said precipitate is a silicagel.

.8. A lubricating grease composition consisting essentially of mineralbase lubricating oil thickened to a grease-like consistency with 1 to20% by dry weight, based on the total composition, of finely dividedsilica gel having its normal water content replaced by said oil, saidgel being waterproofed by adding 0.05 to 2% by weight, based on thetotal composition, of a polystyrene waterproofing agent covering saidgel in a compressed substantially monolayer coating.

9. A lubricating composition consisting essentially of mineral baselubricating oil thickened to a grease-like consistency by 7 /2 to 10% bydry weight, based on the total composition, of silica gel, and 0.1 to 1%of a polystyrene waterproofing agent adsorbed to said gel in acompressed thin layer.

AURELIO F. SIRIANNI. IRA E. PUDDINGTON.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 2,194,683 Badollet Mar. 26, 19402,251,093 Williams July 29, 1941 2,260,625 Kistler Oct. 28, 1941

1. A LUBRICATING COMPOSITION CONSISTING ESSENTIALLY OF MINERAL BASELUBRICATING OIL THICKENED TO A GREASE-LIKE CONSISTENCY BY 1 TO 20% BYWEIGHT, BASED ON THE TOTAL COMPOSITION, OF AN INORGANIC OXYGENCONTAINING GEL VOLUMINOUS PRECIPITATE HAVING ITS NORMAL LIQUID CONTENTREPLACED BY SAID LUBRICATING OIL, AND 0.5 TO 2% OF A POLYSTYRENEWATER-PROOFING AGENT COVERING SAID PRECIPITATE IN T COMPRESSEDSUBSTANTIALLY MONOLAYER COATING TO INHIBIT MOISTURE ABSORPTION BY SAIDPRECIPITATE.